It has one of its composer's most tuneful yet rueful scores, including a couple of his most haunting and heartbreaking songs, “Our Time” and “Not a Day Goes By”. It also has a couple of his most delightfully upbeat and life-affirming songs, too: “Old Friends” and “Opening Doors”.
The show - which revolves around the failure of a professional creative partnership between a composer and a lyricist - is famously the one whose own failure in its original Broadway production in 1981 ended (or, to date, at least suspended) the professional partnership of Sondheim and the director Hal Prince. Together, they'd previously produced such masterpieces of the genre as Company, Follies, A Little Night Music and Sweeney Todd.
But if you read about that original production's troubled gestation (in particular, its use of a very young, totally unknown and theatrically inexperienced cast) and enduring structural difficulties (or at any rate directorial challenges), it becomes less surprising.
Like Harold Pinter's Betrayal (and indeed the original eponymous 1930s Moss Hart play upon which George Furth's book for Merrily is based), the action loops backwards, so that we are shown what the central trio of one-time friends have become in disillusioned middle-age first before we re-trace the steps that brought them there from youthful, optimistic ambition.
Rewinding from 1980 to 1957 as the evening advances, the structure is admittedly problematic because it means the show begins by casting its leading characters (the composer and lyricist and their writer friend) in an unsympathetic light. Our compassion and interest in them grows only as they effectively shed the protective layers that their troubled personal and professional paths have added.
But up-and-coming director Michael Grandage's sympathetic, sharp and subtle production - terrifically energised by the movement of choreographer Peter Darling and the scintillating musical direction of Gareth Valentine - makes the action utterly clear and completely involving. And the superb cast that Grandage has fielded also make real sense of it, connecting emotionally as well as physically.
The central trio - dazzling newcomer Julian Ovenden as the thrusting composer Franklin Shepard around whom the superb Daniel Evans as lyricist Charley Kringas and the simply sensational Samantha Spiro as writer-turned-critic Mary Flynn, revolve - lead a magnificent ensemble. In the cast of 15, there are also terrific contributions from Mary Stockley and Anna Francolini as Franklin's first and second wives.
The show still goes backwards, but in every other respect, the Donmar s brilliant staging is a big step forward in the evolution of this troubled, much travelled but also much loved Sondheim show.